This invention relates to a liquid crystal display element provided with polarizing plates, and more particularly to a field effect type liquid crystal display element.
A liquid crystal display element generally comprises two opposed substrates coated with electrodes and a liquid crystal substance filling the space between the electrodes, and the optical characteristic of the liquid crystal is changed by applying voltage across the electrodes thereby displaying a desired pattern. Such liquid crystal display element has a member of advantageous features that the thickness of the element is small, that the pattern to be displayed can be freely selected, that the power consumption is small, and that the element can be operated at a low operating voltage so that it is possible to directly drive the element by a C-MOS integrated circuit. For this reason, in recent years liquid crystal display elements are abundantly used in wrist watches, table type electronic computers, metering instruments or the like.
Among various types of the liquid crystal display element, a field effect type display element, to which the invention relates, generally comprises polarizing plates mounted on the outer side of two opposed substrates. More particularly, the polarizing plate utilized in this type of the liquid crystal display element generally consists of an organic substance. For example, iodine is adsorbed by a film of polyvinyl alcohol and then the film is elongated in a definite direction to arrange the crystals of the polyvinyl alcohol in a definite direction. The elongated film is then bonded to one side of a cellulose film which acts as a substrate.
However, polyvinyl alcohol is a polymer soluble in water so that it readily dissolves in water in an environment at a high temperature and high humidity thus losing its ability as a polarizing plate. Even immersed in water at room temperature the polarizing ability of a polyvinyl alcohol film degrades greatly in only several hours. For this reason, when liquid crystal display elements incorporated with polarizing plates having such characteristics are used as the display elements of wrist watches, table type electronic computer, metering instruments or the like there occur many problems as follows. These devices are used under various environments including high temperature and high humidity, even under water, which are detrimental to the desired characteristics of the polarizing plate. When used under such adverse environments, the polarizing capability of the polarizing plate is degraded or completely lost, thus rendering inoperative the display element. Such display elements of degraded quality must be discarded.
Considering the field effect type one from different view point, the liquid crystal of the schiff chloride basic or azoxy type loses its liquid crystal property when it is irradiated with ultraviolet rays for a long time. The solar light arriving at the ground surface contains light components having a wavelength longer than 290 n.mu.. For this reason, it was found that schiff chloride basic type liquid crystal loses its liquid crystal property when it is subjected to light components having a wavelength of less than 400 n.mu. for an interval of 1/2 year to 1 year.
In the same manner, the prior art polyvinyl alcohol film used as the polarizing plate degrades its property due to ultraviolet rays, so that this plate is not satisfactory.